The present invention relates generally to printing machines, and more particularly to an apparatus and method for controlling the ink feed in such machines.
In sheet-fed offset printing machines, the ink feed is carried out from an ink fountain, interacting with an ink fountain roll and ink metering elements. Zonally metered ink on the ink fountain roll is transferred via a doctor roll to a first distributor roll and, from there, via a number of rolls to a plurality of ink applicator rolls set against the plate cylinder.
In order to achieve a uniform ink layer thickness application to a printing plate clamped on the plate cylinder and, in this way, to obtain an ink density which is uniform over the print length, the front and rear ink applicator rolls in the printing direction each are connected to a respective roll train. Such a two-train inking unit is shown, for example, by DE 40 13 740 C2.
When rotational speeds of the printing cylinders are increased, in particular following production interruptions, the first printed sheets normally exhibit clearly visible overinking. In the case of printed subjects with a high ink coverage area this effect is especially pronounced. In order to counteract such overinking, attempts often are made to reduce the ink feed, which can be done for example by closing the ink metering elements or stopping the doctor roll. However, these measures require a lead time which must be coordinated exactly with the ink flow and the printing speed, since the inertia of the inking unit is relatively high. That is to say, the interruption to the ink flow is effected only after some machine revolutions.
Inking units of sheet-fed offset printing machines frequently also have what are known as ink flow separators which can interrupt the roll train at one or more points. Such ink flow separators are used during printing interruptions to prevent the ink layer thickness profile from breaking down as the inking unit continues to run. When printing is started, however, the rolls are set against one another again so that the original ink flow again is produced.
DE 36 40 295 C2 discloses a method for avoiding the overinking of inking units on printing machines, in which, during the printing operation, an ink applicator roll is thrown off, separated from the roll train and washed during this phase. In this way, from time to time excess ink from the inking unit can be dissipated.
It is an object of the invention to provide a method and apparatus for more accurately controlling the ink feed in a printing machine.
Another object is to provide a method and apparatus as characterized above that is effective for preventing overinking of printing material when the speed of the printing machine is increased, such as during start up of the printing operation.
According to the invention, in a phase in which the printing speed is increased, provision is made for interrupting the roll train at least for some time, at at least one point. As a result of this interruption of the roll train, an excess of ink built up in the inking unit, for example as a result of a previous print interruption, cannot produce overinking on the printed sheets.
According to one embodiment of the invention, in an inking unit that supplies the applicator rolls with ink via two roll trains, the two roll trains are divided or interrupted at at least one point as the rotational speed is increased. The dividing point, which may be in a lower part of the inking unit, results in a specific measure and application that can be coordinated accurately with the printing conditions in order to avoid overinking.
According to a further embodiment of the invention, in an inking unit supplying the applicator rolls with ink via two roll trains, the ink supply of the front ink applicator rolls in the printing direction remains unchanged, while provision is made for briefly uncoupling the rear ink applicator rolls during the acceleration phase. This corresponds to a brief changeover to what is known as a frontloaded inking unit, in which the main quantity of ink fed in is applied on the first or the first two ink applicator rolls (as viewed in the direction of rotation of the plate cylinder). In this case, the method according to the invention is preferably carried out during the production start-up phase, in which the printing speed also is increased.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: